In overt hyperthyroidism, serum levels of free T4 and triiodothyronine (T3) or levels of T3 alone are elevated, and serum thyrotropin (TSH) levels are suppressed. In subclinical hyperthyroidism, levels of free T4 and T3 are normal, thyrotropin levels are suppressed, and thyroid hormone levels are usually in the middle to upper range of normal.
The diagnosis of subclinical hyperthyroidism is based on laboratory results, but several other common clinical situations are associated with similar laboratory findings.
Overt primary hyperthyroidism
Suppressed thyrotropin levels and elevated levels of free thyroxine (T4) and triiodothyronine (T3) or elevated levels of T3 only
In mild cases, low but detectable serum thyrotropin levels (0.1 to 0.4 mU per liter) with normal levels of free T4 and T3
In severe cases, undetectable serum thyrotropin level (<0.1 mU per liter) with normal levels of free T4 and T3
Other causes of low serum thyrotropin levels
The following causes of low serum thyrotropin levels should be ruled out before a diagnosis of subclinical hyperthyroidism is made:
- Severe nonthyroidal illness
- Administration of drugs that suppress serum thyrotropin levels (e.g., dopamine, high doses of glucocorticoids, dobutamine, somatostatin analogues, amphetamines, bromocriptine, and bexarotene)
- Pituitary or hypothalamic disease that causes thyroid hormone or thyrotropin deficiency
- Psychiatric illness
- Late first-trimester of pregnancy
- Hyperemesis gravidarum
- Older age (i.e., age-induced changes in the hypothalamic–pituitary thyroid axis in areas of the world with iodine deficiency)
- African descent (thyrotropin levels are below the reference range in 3 to 4% of patients)
- Biondi B, Cooper DS. Subclinical Hyperthyroidism. N Engl J Med. 2018 Jun 21;378(25):2411-2419. [Medline]
- De Leo S, Lee SY, Braverman LE. Hyperthyroidism. Lancet. 2016 Aug 27;388(10047):906-918. [Medline]
Created Aug 07, 2018.